Where impurities contained in a semiconductor are measured, it is necessary to maintain a sample of the semiconductor at an appointed temperature within the measurement facility, and to control the temperature between a high temperature and a low temperature. Conventionally, a cryostat as shown in FIG. 2 has been used.
Referring to the cryostat shown in FIG. 2, reference numeral 1 designates a vacuum chamber internally insulated from the air. The chamber 1 is provided on a suitable base 2 and depressurized to an appointed extent by a vacuum exhauster (not shown).
The vacuum chamber 1 is provided internally with a sample table 3 for holding a sample (not shown), such as a semiconductor, and a refrigerator 4 for cooling the sample table 3. The vacuum chamber 1 is also provided with a closing sample port (not shown) formed in the vicinity of the sample table 3, for placing the sample within the chamber 1 and removing the sample.
The refrigerator 4 is a two-stage refrigerator comprising a first-stage refrigerator 5 and a second-stage refrigerator 6. The second stage refrigerator 6 is provided over the first-stage refrigerator 5 and connected with the first-stage refrigerator 5, such- that the second-stage refrigerator 6 is cooled to an appointed temperature by the first-stage refrigerator 5. Likewise, the sample table 3 is cooled to an appointed temperature by the second-stage refrigerator 6.
The first-stage refrigerator 5 is connected with a compressor, a helium gas-storing medium, a pump, and the other refrigeration mechanics (not shown) through pipings 7, 8.
A heat conductor 9 having a high heat conductivity is provided over the second-stage refrigerator 6. The sample table 3 is held over the heat conductor 9 so as to be thermally connected with the refrigerator 4 through the heat conductor 9.
The heat conductor 9 is provided with a heater 10 wound therearound for controlling a temperature of the sample table 3. The sample table 3 is provided with a temperature sensor 11 for detecting the temperature of the sample table 3.
Reference numeral 12 designates a temperature-controlling apparatus provided outside of the vacuum chamber 1. The temperature controlling apparatus 12 maintains the temperature of the sample table 3 at an appointed temperature by causing heat to be emitted from the heater 10. The level of heat emitted is determined on the basis of the signal from the temperature sensor 11.
When the sample is exchanged from the chamber, the vacuum chamber 1 is brought into an open-air condition and then the sample port is opened. However, if the temperature of the sample table 3 and the refrigerator 4 are approximately 0.degree. C. or less, they may become dewed. This is an unwanted and dangerous condition. It is therefore necessary to raise the temperature of the sample table 3 and the refrigerator 4 up to a temperature at which they are not dewed before the vacuum chamber 1 is brought into an open-air condition.
However, this conventional cryostat is only provided with the heater 10 for controlling the temperature of the sample table 3. Furthermore, the single heater 10 has a small calorific power. Therefore, the temperature of the sample table 3 and the refrigerator 4 can only be raised very slowly.
Additionally, because the refrigerator 4 is a two-stage refrigerator, it takes a longer time, in comparison with the sample table 3, to raise the temperature of the refrigerator 4 and, in turn, the system.